The field of this invention relates to automatic welding apparatuses and more particularly to a welding apparatus which is designed primarily to facilitate welding of tubes to a tube sheet.
A heat exchanger typically consists of a shell in which are located a large number of parallel tubes for carrying cooling liquid. Steam entering the shell condenses on the tubes. At both ends of the heat exchanger, the tubes are held in position by a tube sheet. These spaced apart tube sheets separate the steam containing shell interior from the remaining portion of the heat exchanger.
In a typical heat exchanger, there may be literally hundreds in number of tubes mounted in a closely spaced arrangement within a tube sheet. It is necessary to insure a leakless joint between each tube and the tube sheet. Rolling or expansion of the tubes into the tube sheet does not provide insurance of a leakless joint. Therefore, generally it is desirable that the tubes be welded to the tube sheet.
It has been found that by manual welding of each of the tubes to the tube sheet, insufficient quality is obtained. The inherently repetitive creating of each annular weld results in the production of an occasional poor quality weld. Even if a single tube is found to leak, that leak must be repaired. Therefore, some means must be utilized to achieve a hundred percent weld quality at every weld.
In order to achieve this hundred percent weld quality, it is common to use an automatic welding apparatus. The automatic welding apparatus is securable in a set position with respect to the welding area. Desirable weld quality is obtained through the use of an inert gas environment to the tip of the welding electrode. The welding current and voltage is monitored through the use of a computer and instantaneous changes will be made if preset desirable parameters are not met during the welding procedure.
It has been common in the past to design a type of welding head which mounts directly within the tube that is being welded. This type of mounting utilizes some form of a mandrel assembly which extends within the tube. The mandrel assembly is to include a series of balls or other similar type of structure which is to be biased outwardly into contact with the wall of the tube thereby fixing the welding head in position. However, in the past, it has been found that this securing of the mandrel to the tube has not be sufficiently positive. Usually some form of a biasing spring arrangement is utilized which exerts the desired force to extend the members in contact with the wall surface of the tube. Inherently, a biasing spring arrangement has "give" to it. This "give" permits ever so slight movement of the welding head during the welding operation. This slight movement diminishes weld quality.
Also, prior art tube sheet welding apparatuses utilize one or two gas passages to supply inert gas to the tip of the welding electrode. It has been found that the supply of the inert gas tends to stream, which result in uneven dispersement of the inert gas. This uneven dispersement again diminishes weld quality.
Additionally, the welding temperature that is created is suficiently high which causes the welding head housing to substantially raise in temperature. It is desirable to include some type of heat dissipation structure within the welding head housing in order to eliminate this undesired accumulated heat. An excessively hot welding head housing makes it difficult for the operator to physically grasp the welding head housing and move such from one tube location to another tube location. Such movement is necessary since there is generally hundreds of welds to be completed within a single tube sheet.